Area Forecast Discussion
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FXUS63 KLOT 270202
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
902 PM CDT THU MAR 26 2015

.UPDATE...
902 PM CDT

NO BIG CHANGES PLANNED TO GOING FORECAST TONIGHT INTO FRIDAY
MORNING. DID NUDGE TEMPERATURES UP ABOUT A CATEGORY ACROSS THE
BOARD AS CLOUD COVER AND WINDS LOOK TO LIMIT RADIATIONAL COOLING
AND COLD ADVECTION LIKELY REMAINING FAIRLY WEAK DUE TO THE FAIRLY
WEAK TEMPERATURE GRADIENT UPSTREAM.

REGARDING LAKE EFFECT POTENTIAL...SUBTLE SFC TROUGH IS MOVING
SOUTH DOWN THE LAKE THIS EVENING WITH TENDENCY FOR MEAN BOUNDARY
LAYER FLOW BEHIND THIS TROUGH TO VEER TO NORTH-NORTHEASTERLY.
ALREADY SEEING SOME ISOLATED MORE ROBUST LAKE EFFECT SNOW SHOWERS
BLOSSOMING NE OF MKE AND WITH THERMODYNAMICS LIKELY TO ONLY
IMPROVE THROUGH THE NIGHT WOULD EXPECT SNOW SHOWER COVERAGE TO
ALSO INCREASE. HI-RES MODEL GUIDANCE SUGGESTS A 3-6 HOUR WINDOW OF
MEAN FLOW VEERING TO THE EAST OF DUE NORTH FROM AROUND MIDNIGHT
UNTIL SUNRISE WHICH COULD BEGIN SENDING LAKE EFFECT SNOW SHOWERS
ONSHORE INTO NE IL AND CHICAGO METRO AREA. GIVEN THE SOMEWHAT
MARGINAL TEMP DIFFERENTIAL BETWEEN LAND AND MARINE LAYER DOESNT
APPEAR A STRONG LAND/LAKE CONVEREGENCE ZONE WILL DEVELOP...SO
PROBABLY LOOKING FOR MAINLY SCATTERED AND SOMEWHAT LOOSELY
ORGANIZED SNOW SHOWERS WHICH WILL HELP LIMIT ACCUMULATIONS. IN
ADDITION...PAVEMENT TEMPS ARE IN THE 40S THIS EVENING AND WITH
TEMPS STILL ABOVE FREEZING IT APPEARS MUCH OF THE SNOW WOULD
INITIALLY MELT ON ROADS CONFINING ACCUMULATIONS TO ELEVATED AND
GRASSY SURFACES. EVEN SO...FORECAST SOUNDINGS SHOW A FAIRLY DEEP
MOIST ABSOLUTELY UNSTABLE LAYER DEVELOPING OVERNIGHT SO AT LEAST
ISOLATED HEAVIER SNOW SHOWERS COULD RESULT IN LOCALIZED ACCUMS OF
A COATING TO AROUND AN INCH. WILL BEEF UP POPS SLIGHTLY IN NE IL
AFTER MIDNIGHT AND GO WITH SCATTERED WORDING VS CHANCE AS PROB OF
SNOW SHOWERS BEING AROUND APPEARS SUFFICIENTLY HIGH.

IZZI

&&

.SHORT TERM...
226 PM CDT

THROUGH FRIDAY...

MAIN CONCERN IN THE SHORT TERM IS LAKE EFFECT PRECIP THIS EVENING
INTO FRIDAY. AN UPPER LEVEL TROUGH CURRENTLY OVER THE CENTRAL CONUS
SHIFTS EAST WITH ITS AXIS MOVING THROUGH THE REGION FRIDAY MORNING.
AT THE SURFACE AN ELONGATED REGION OF LOW PRESSURE IS OVER THE
APPALACHIANS AND IT MOVES OVER THE ATLANTIC OCEAN FRIDAY AFTERNOON.
HIGH PRESSURE IS OVER MANITOBA AND IT MOVES OVER THE REGION FRIDAY
EVENING.

AS THE HIGH MOVES CLOSER...WINDS BECOME NORTH DOWN THE LAKE
RESULTING IN FAVORABLE CONDITIONS FOR LAKE EFFECT PRECIP.  GUIDANCE
SLOWS THE VEERING OF THE WINDS SO PUSHED BACK THE ARRIVAL OF LAKE
EFFECT PRECIP.  THERE IS LESS SHEAR IN THE FORECAST SOUNDINGS WHICH
SUGGESTS IT SHOULD BE EASIER TO CREATE A SINGLE BAND OF LAKE EFFECT
PRECIP.  HAVE HIGH CONFIDENCE IN THE AREAS ALONG THE LAKE SEEING
SOME SNOW...BUT MEDIUM CONFIDENCE IN THE TIMING AND LOCATION OF THE
BAND. ALSO HAVE MEDIUM CONFIDENCE IN SNOW TOTALS AND WENT MIDDLE OF
THE ROAD FOR SNOW AMOUNTS AT THIS TIME.

FOR THE IL SHORE...EXPECTING LAKE EFFECT PRECIP TO PUSH ON SHORE
AROUND OR AFTER 10PM WITH THE BEST CHANCE FOR PRECIP AFTER MIDNIGHT.
 THINKING THE MAJORITY OF PRECIP WILL FALL AFTER 3AM WITH THE AREA
OF SNOW SLOWLY SHIFTING EAST.  WINDS SHOULD TURN NORTH ENOUGH TO
PUSH THE SNOW OVER NW INDIANA ARND 10AM.  THE INITIAL PRECIP MAY
FALL AS A RAIN/SNOW MIX AS WE WILL BE AROUND FREEZING AT THE SFC AND
LACKING SATURATION IN THE DGZ...BUT WE SHOULD MOISTEN UP NICELY
LEADING TO A QUICK TRANSITION OVER TO SNOW.  NOT EXPECTING ANY
SIGNIFICANT OR IMPRESSIVE SNOW TOTALS...BUT THINKING UP TO A HALF OF
AN INCH OF NEW SNOW WILL FALL.  LATEST MODEL RUNS ARE HINTING AT
ENHANCED PARAMETERS FOR LAKE EFFECT SNOW ALONG THE IL SHORE BUT DO
NOT WANT TO JUMP ON THAT BANDWAGON JUST YET SO BUMPED SNOW TOTALS UP
A BIT TO TREND IN THE DIRECTION OF THE LATEST GUIDANCE.

FOR THE IN SHORE...ALSO EXPECTING THE BAND TO REACH LAND AROUND 10
PM AND INTENSITY TO PICK UP AFTER MIDNIGHT.  SOME MODELS FEATURE A
RIBBON OF CONVERGENCE THAT COULD LEAD TO A BAND OF MODERATE
SNOWFALL...BUT ONCE AGAIN STILL A BIT TOO SOON TO GET TOO SPECIFIC.
SNOW WILL SLOWLY SHIFT EAST OVER NW INDIANA THROUGH FRIDAY MORNING.
SOUNDINGS FEATURE QUITE A BIT OF FORCING...BUT SLIGHTLY BELOW THE
DGZ...SO WENT A BIT CONSERVATIVE WITH SNOW AMOUNTS.  THINKING THE
LONGER FETCH AND TIME UNDER THE BAND WILL LEAD TO AROUND AN INCH OF
NEW SNOW. LOCALLY HIGHER TOTALS ARE CERTAINLY POSSIBLE IF A SINGLE
BAND OF SNOW FORMS.

COLDER AIR SINKS SOUTH AS WELL FRIDAY MORNING WITH HIGHS ONCE AGAIN
WELL BELOW NORMAL IN THE LOW TO MID 30S. HIGH PRESSURE MOVES
OVERHEAD FRIDAY AFTERNOON WITH SKIES CLEARING AND NORTH WINDS
DIMINISHING. A CHILLY NIGHT IS EXPECTED WITH CLEAR SKIES AND WEAK
WINDS. LOWS WILL BE IN THE TEENS WITH A LOW AROUND 20 DOWNTOWN.

JEE

&&

.LONG TERM...
226 PM CDT

FRIDAY NIGHT THROUGH THURSDAY...

HIGH PRESSURE SITS OVER THE REGION FRIDAY NIGHT AND SATURDAY BEFORE
SHIFTING SOUTHEAST OV THE OHIO VALLEY SATURDAY EVENING.  UPPER LEVEL
TEMPS SLOWLY WARM SATURDAY BUT STILL LOOKING AT BELOW NORMAL TEMPS
WITH HIGHS IN THE UPPER 40S AND LOWS IN THE 20S.

THE NEXT UPPER LEVEL TROUGH MOVES OVER SW CANADA SATURDAY AFTERNOON
AND THEN PASSES OVER THE REGION SUNDAY AFTERNOON AND EVENING.  THE
SFC LOW ASSOCIATED WITH THE TROUGH PASSES OVER ONTARIO SUNDAY AS
WELL...AND GUIDANCE IS MUCH MORE CONSISTENT WITH THE LOW/S TRACK
OVER ONTARIO.  GUIDANCE DOES DIFFER WITH THE SPEED OF THE LOW AND
ITS COLD FRONT...WITH THE EURO BEING QUICKER WITH THE LOW YET SLOWER
WITH THE COLD FRONT.  THE NAM AND GFS WERE ON THE SAME PAGE TIMING
WISE SO LEANED HEAVILY IN THEIR DIRECTION.  THEREFORE HAVE PRECIP
MOVING IN SUNDAY MORNING AND PUSHING EAST OF THE CWA SUNDAY EVENING.
FORECAST SOUNDINGS INDICATE THE INITIAL PUSH OF PRECIP MAY BE A MIX
OF SNOW AND SLEET WITH A NICE WARM NOSE ALOFT. PRECIP THEN
TRANSITIONS TO RAIN. FOR HIGH TEMPS...WENT A BIT ON THE COOLER SIDE
SINCE THERE WILL BE AMPLE CLOUD COVER. HAVE HIGH TEMPS IN THE 40S
AND THE GUSTY SOUTHWEST WINDS WILL CERTAINLY MAKE IT FEEL BRISK.

A WEAK SURFACE RIDGE QUICKLY SLIDES PAST MONDAY WITH THE NEXT WEAK
LOW MOVING OVER CENTRAL WI MONDAY NIGHT INTO TUESDAY MORNING.
GUIDANCE IS A BIT MORE PERSISTENT WITH PRECIP BUT THE EURO IS STILL
THE ONLY MODEL BRINGING PRECIP OVER THE CWA. THEREFORE CAPPED POPS
IN THE SLIGHT CHANCE RANGE.  HIGH TEMPS MONDAY AND TUESDAY WILL BE A
BIT MORE SEASONAL IN 50S AND MAYBE AROUND 60 TUESDAY AWAY FROM THE
LAKE.

THE MODELS DISAGREE WITH THE TIMING OF THE PATTERN MID TO LATE NEXT
WEEK WITH A BROAD HIGH MOVING OVERHEAD MID WEEK WITH ANOTHER LOW
PRESSURE SYSTEM LATE IN THE WEEK.  THE LOW WILL BRING ANOTHER CHANCE
OF RAIN AND TEMPS LOOK TO BE AT LEAST AROUND NORMAL IF NOT ABOVE
NORMAL WITH A RELATIVELY ZONAL PATTERN ALOFT.

JEE

&&

.AVIATION...

//ORD AND MDW CONCERNS...UPDATED 00Z...

* NORTHWEST WINDS TURNING NORTH/NORTHEAST LATE THIS EVENING.
* LAKE EFFECT SNOW SHOWERS OVERNIGHT.
* GUSTY NORTH WINDS FRIDAY...SHIFTING NORTHEAST WITH A POSSIBLE
  LAKE BREEZE FRIDAY AFTERNOON. CMS

//DISCUSSION...UPDATED 00Z...

HIGH PRESSURE WILL SLOWLY BUILD ACROSS THE WESTERN GREAT LAKES
REGION OVERNIGHT INTO FRIDAY. AS IT DOES...WINDS WILL TURN FROM
NORTHWEST TO MORE NORTHERLY THIS EVENING AND THEN EVENTUALLY TO
THE NORTH/NORTHEAST OVERNIGHT. AS THIS OCCURS...LAKE EFFECT SNOW
SHOWERS ARE EXPECTED TO DEVELOP THIS EVENING AND THEN SPREAD
ACROSS NORTHEAST IL AND NORTHWEST INDIANA OVERNIGHT...PERHAPS
LASTING INTO FRIDAY MORNING. CONFIDENCE REGARDING OVERALL COVERAGE
AND DURATION IS LOW. HOWEVER...IFR OR EVEN BRIEF LIFR CIGS/VIS
WILL BE POSSIBLE WITH ANY HEAVIER LAKE EFFECT SNOW SHOWERS.
MAINTAINED PREVAILING MVFR VIS/CIGS FOR NOW AND MAY NOT BE ABLE
TO NARROW DOWN A TIME RANGE OR CONDITIONS UNTIL SNOW SHOWERS
DEVELOP. WINDS WILL TURN MORE NORTHERLY FRIDAY MORNING AND LIKELY
BECOME GUSTY...INTO THE THE UPPER TEENS/NEAR 20KT RANGE. BUT AS
THE HIGH CONTINUES TO BUILD ACROSS THE REGION...A LAKE BREEZE WILL
PROBABLY DEVELOP...SHIFTING WINDS BACK TO NORTHEAST AT ORD/MDW.
CMS

//ORD AND MDW CONFIDENCE...UPDATED 00Z...

* MEDIUM FOR WINDS THRU THE PERIOD.
* MEDIUM FOR LAKE EFFECT SNOW SHOWERS OVERNIGHT...LOW FOR DURATION
  AND ASSOCIATED CIGS/VIS. CMS

//OUTLOOK FOR ORD/MDW FOR 12Z SATURDAY-12Z THURSDAY...UPDATED 00Z...

* SATURDAY...DRY/VFR. POSSIBLE LAKE BREEZE.
* SUNDAY...RAIN LIKELY. MVFR LIKELY. STRONG/GUSTY SOUTHWEST WINDS.
* MONDAY...DRY/VFR. WEST WINDS.
* TUESDAY...DRY/VFR. POSSIBLE LAKE BREEZE.
* WEDNESDAY...CHANCE OF RAIN. MVFR POSSIBLE. SOUTH WINDS.

&&

.MARINE...
203 PM CDT

MAIN MARINE FORECAST CONCERNS FOCUS ON A COUPLE OF PERIODS OF
STRONGER WINDS...THE FIRST FROM THE NORTH LATE TONIGHT THROUGH
FRIDAY EVENING AND THE SECOND A STRONGER PERIOD OF SOUTH WINDS AND
POTENTIAL GALES SATURDAY NIGHT THROUGH SUNDAY AND THEN BECOMING
WEST AND VERY SLOWLY DIMINISHING SUNDAY NIGHT INTO MONDAY.

LOW PRESSURE WAS LIFTING NORTHEAST THROUGH THE OHIO VALLEY THIS
AFTERNOON...WHILE HIGH PRESSURE WAS PUSHING SOUTHWARD FROM CENTRAL
CANADA. IN ADDITION...A WEAK SURFACE TROUGH WAS PUSHING SOUTH
ACROSS THE LAKE...WITH COLDER AIR AND A TIGHTENING GRADIENT AHEAD
OF THE APPROACHING HIGH STARTING TO SHIFT WINDS MORE TO THE NORTH-
NORTHWEST WITH GUSTS NEAR 30 KT ON THE NORTH END OF THE LAKE. THIS
TROUGH WILL CONTINUE TO MOVE SOUTH DOWN THE LAKE THROUGH THIS
EVENING...WITH WINDS BECOMING NORTHERLY AND INCREASING TO 30 KT
TONIGHT AND SLOWLY EASING TO 25 KT OR SO THROUGH LATE FRIDAY. THIS
NORTHERLY FETCH WILL LEAD TO WAVES BUILDING INTO THE 8 FOOT
RANGE...AND WILL HAVE A SMALL CRAFT ADVISORY IN EFFECT AFTER 06Z
FRI/1 AM CDT FOR THE IL/IN NEARSHORE WATERS. WINDS WILL CONTINUE
TO DIMINISH MORE QUICKLY LATER FRIDAY NIGHT AND SATURDAY...THOUGH
WILL HAVE THE SCA RUN THROUGH 21Z SAT/4 PM CDT AS WAVES SUBSIDE
MORE SLOWLY.

THE SURFACE HIGH WILL CONTINUE TO SPREAD SOUTHEAST ACROSS THE
WESTERN LAKES AND INTO THE OHIO VALLEY BY SATURDAY EVENING...
ALLOWING WINDS TO SHIFT BACK TO THE SOUTH. A DEEPENING AREA OF LOW
PRESSURE IS EXPECTED TO DEVELOP AND MOVE EAST THROUGH MANITOBA AND
INTO ONTARIO SATURDAY NIGHT INTO SUNDAY...TIGHTENING THE GRADIENT
AND INCREASING SOUTH WINDS TO 35-40 KT GALES BY SUNDAY MORNING.
WINDS LOOK TO EAST A BIT LATE SUNDAY AND SHIFT WEST AS THE
TRAILING COLD FRONT MOVES ACROSS THE LAKE. WESTERLY WINDS THEN
CONTINUE TO DIMINISH MONDAY.

SOUTH GALES APPEAR TO BE FAIRLY LIKELY FOR DURING THE DAY
SUNDAY... THOUGH ON DAY 4 STILL A LITTLE BEYOND THE TIME PERIOD
FOR A GALE WATCH. IF FORECAST GUIDANCE CONTINUES TO DEPICT A
SIMILAR SCENARIO WILL LIKELY NEED A GALE WATCH IN THE NEXT DAY OR
SO.

RATZER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...1 AM FRIDAY TO 4 PM SATURDAY.

&&

$$

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